The present invention relates to isocyanate-reactive compositions and process for preparing a flexible polyurethane foam using phenolic antioxidants. More particularly, this invention concerns high molecular weight phenolic antioxidants and their use in flexible polyurethane foams.
Flexible polyurethane foams, one of the largest segments of polyurethane foams, are useful for a wide range of applications. Their most common uses are in cushioning, as in furniture and automobiles, bedding, carpet backing, and other padding or cushioning applications.
Polyurethane foams have been conventionally prepared by reacting a high equivalent weight isocyanate-reactive compound, a polyisocyanate in the presence of a blowing agent. It is also well-known that useful blowing agents include, for example, air, water, low boiling liquids, such as chlorofluorocarbon, or mixtures thereof. In preparing flexible polyurethane foams, water has been the preferred blowing agent.
A persisting problem, however, in the preparation of flexible polyurethane foams, especially in slabstock-type foams, is foam polymer degradation resulting in discoloration (herein also referred to as "scorch").
Scorch is a well-known thermoxidative process caused by the heat released from the exothermic reaction between water and the isocyanate. This thermoxidative process may be further exacerbated by the ambient heat and humidity conditions and can reach levels of self-ignition of the foam. Therefore, scorch is considered one of the most serious issues since it represents a potential fire hazard for the foam manufacturers. Scorch is normally expressed as a function of coloration of the foam which is expressed as .DELTA.E. The higher .DELTA.E, the higher the scorch of the foam.
Due to the growing efforts to reduce the use of chlorofluorocarbons (CFC) blowing agents in the foam formulations by substituting part of it with water, thereby increasing the overall water content scorch has been more frequently addressed. These foam formulations may contain up to about 6 or more parts of water per hundred parts (pph) of isocyanate-reactive compound instead of the usual 2 to 3 pph of water and are commonly called high water formulations.
Scorch, which usually appears as a faint to severe discoloration in the center of a foam, is more likely to occur in slabstock foams. Due to the low thermal conductivity of the foamed materials, the dissipation of the heat occurs only very slowly. Thus, the interior of a slabstock foam bun can reach temperatures between about 140.degree. C. to about 170.degree. C., thereby contributing to the polymer degradation or discoloration and is more frequently encountered in high water slabstock foam formulations. The higher the water content in the foam formulation the more susceptible it is to the scorch phenomena. In addition to reducing the aesthetic value of the foam due to the discoloration to the point of the product being considered scrap or without any commercial value, scorch adversely affects several important physical properties of the foam, such as, for example, tensile strength, tear strength, elongation and compression set.
It is also well-known that the thermoxidative process or degradation process is caused by the free radical mechanism and can be controlled to a certain degree by adding antioxidants to the foam formulations in order to reduce their degradation. In flexible polyurethane foams, for example, antioxidants are normally added to the isocyanate-reactive compound, prior to the foaming step, in order to reduce foam discoloration and loss of physical properties. However, the discoloration problem is still encountered to some extent in the resultant slabstock foams.
In view of the above mentioned deficiencies, efforts are being made to ways to reduce or even eliminate the scorch phenomenon in flexible polyurethane foams.
Currently, scorch in flexible foams is being controlled by adding low molecular weight hindered phenolic antioxidants, i.e., molecular weight of from about 200 to about 550, in conjunction with other inhibitors to the formulation. U.S. Pat. Nos. 4,007,230; 4,265,783; 4,275,173 and 4,363,745 to Hinze et al., for example, cover flexible polyurethane foams prepared from polyols stabilized with the above-mentioned low molecular weight antioxidants. Unfortunately, all these efforts were not sufficient to effectively control and/or eliminate the scorch problem.
Higher molecular weight antioxidants, i.e., molecular weights above 550, have been disclosed, for example, in U.S. Pat. Nos. 3,644,482 and 3,944,594. These patents relate to hindered phenolic esters of oligomeric glycols and thioglycols used as stabilizer in plastic materials, such as polypropylene, polyurethane, polyacetals, nylon, EPDM rubber, SBR rubber and polybutadiene.
U.S. Pat. No. 3,722,218 discloses a stabilizer system comprising a primary antioxidant, such as a high molecular weight hindered phenolic compound, a second high molecular weight phosphite antioxidant and an ultraviolet absorbing compound. However, the hindered phenolic antioxidants, taught as having high molecular weights, present a molecular of 520 or less. This system is useful in flexible, open-cell polyurethane foam.
U.S. Pat. No. 4,677,154 relates to a stabilizer package for polyurethanes comprising a substituted cresol and a high molecular weight phosphorus compound.
However, none of the above described references achieves a satisfying control and/or reduction of the scorch phenomena. It would, therefore, be most desirable to provide an effective antioxidant for use in the preparation of polyurethane foams so as to significantly reduce scorch.